Once you obtain the eyes, give it to the fire keeper. At half health he will go into his second phase, signified by an increase in his speed and also the blue glow in his eyes.

If you are just looking to get this trophy, then you can run to the Archdeacon. Once you have obtained it you must head towards the large painting in the room with the high beams in Anor Londo, this will grant you access into the Painted World. But i can find this walkthrough to get the platinum with a minimum effort?

The others will also try to help speed the attack time. Just wanted to point out that you can still join Blue Sentinels if you miss Horace the first time. She has a vertical sword smash that also is a small fiery explosion, and variations to her horizontal sweeps.